With the continuous increase of environmental controls over the discharge of certain gases, such as sulfur dioxide, into the atmosphere, the development of more efficient and economical means for removing such gases from flue gases has steadily continued. Especially in coal burning power plant systems for production of electricity, restrictions are applicable which require an ever increasing efficiency in removal of sulfur dioxide from flue gases.
Sulfur dioxide removal systems are generally characterized as (a) dry scrubbing systems, where a solution or slurry of a reagent is dispersed in hot flue gases, with the water content thereof evaporated, and collection of relatively dry by-products made for disposal, or (b) wet scrubbing systems, were an aqueous solution or slurry of a reagent, such as an alkali or alkaline earth reagent is contacted with the gases, with collection and disposal of wet sludges.
Dry scrubbing systems are generally classified into two categories. One is the dry sprayer or vertical reactor where the sorbent is dispersed into a flow of sulfur dioxide-containing flue gases, with optional removal of fly ash prior to the contact. The other is the dry sorbent injection method where a sorbent, with possible accelerators, is injected into a horizontal ductwork through which the gases are passed with dry solid reactants and excess sorbent removed from the gas prior to discharge to the atmosphere. The vertical reactor type dry system is suited primarily for use with lower sulfur dioxide content gases and provides reasonable percentage removals, while the dry sorbent injection type dry system generally gives lower percentages of sulfur dioxide removal from the gases.
Wet scrubbing systems have also evolved into two main categories, those using a vertical tower and those using a horizontal contactor. In the vertical tower scrubbing systems, gas is normally charged to the tower at a lower portion thereof and passes upwardly through the tower in contact with a countercurrently flowing dispersed aqueous scrubbing solution or slurry. Spaced contact trays may be provided in the tower to achieve better contact of the scrubbing medium and the gas, while a demister or liquid removal device is provided near the top of the vertical scrubber to remove liquid from the gas stream prior to discharge of the cleansed gas stream to the atmosphere. Difficulties inherent in the use of such vertical scrubbers include inadequate or sporadic contact of the gaseous stream with the descending aqueous scrubbing media, an increased pressure drop within the scrubbing unit as the gases flow therethrough, and the need to assure that all liquid is removed from the gases prior to discharge from the upper region of the vertical scrubber. In order to assure such complete removal of liquid, prior to discharge of the gas, the volume and rate of flow of the gases through the scrubber is limited.
In order to avoid problems associated with vertical scrubbers, horizontal contactors or scrubbers have been developed. As an example of such horizontal scrubbers, reference may be made to U.S. Pat. No. 3,948,608 to Weir, the contents of said patent incorporated by reference herein. In U.S. Pat. No. 3,948,608, an apparatus for wet scrubbing of gases is described which uses an elongated chamber defining a substantially horizontally extending flow path for gases to be cleaned having a substantially unrestricted cross-sectional flow area along its length so as to avoid any pressure drop between an inlet to the chamber and an outlet from the chamber. Liquid reagent sprayers are provided that spray reagent across the chamber substantially perpendicular to the flow path of the gases to remove sulfur dioxide from the gases by the spray droplets, and spent reagent is collected and discharged from the bottom of the chamber. The collected spent reagent is collected in a tank as a liquid slurry. Improvements on this original system are described in subsequent patents to Weir, such as U.S. Pat. Nos. 4,102,982 and 4,369,167, while a further improvement in the process is described in U.S. Pat. No. 4,269,812 to Edwards and Huang. Such prior art horizontal scrubbers have been limited by the gas velocity that can be accommodated while providing a high removal of sulfur dioxide. At the present time, all known flue gas desulfurization wet scrubbing systems being offered commercially are of the vertical tower type due to the limitations associated with horizontal scrubbers.
It is an object of the present invention to provide a horizontal scrubber and method for removal of sulfur dioxide from flue gases which provides removal of at least ninety percent of the sulfur dioxide while providing a high flue gas velocity through the scrubber.